lo and yo

Literally, lo sé means I know it, whereas yo sé means I know. But practically, I think they can be used interchangeably. Perhaps lo sé could be used when you want to emphasize what you know...I know IT, whereas yo sé could be used when you want to emphasize that you know whatever is known...I know.

Saying "yo siento" probably does not mean what you think it does. You have to say "lo siento" to effectively say I'm sorry. The verb "sentir" means "to feel" so saying "lo siento" literally means "I feel it" and that somehow means "I'm sorry". It would be strange to simply say "I feel" since the verb seems to demand a subject.

Definitely: "Lo siento" is the formula way to I'm sorry. An interesting question is when you would use "sentir" as opposed to "sentirse". I would venture the following:
- siento que voy a poder...:
- me siento [adjective]

You'll be amazed at how much Spanish you'll learn with the most comprehensive Spanish system there is１番包括的なシステムで驚くほど英語を習いましょうTe sorprenderás de cuánto inglés aprenderás con el sistema de inglés más completo que existe

You get lifetime access to hours of selected lessons, with voice recognition tools to perfect your pronunciation数時間の選択レッスンの一生アクセス。音声認識ソフトで発音を完璧に！Obtienes acceso de por vida a horas de lecciones seleccionadas, con herramientas de reconocimiento de voz para perfeccionar tu pronunciación